A diffusion model for the time-dependent transport of dust undergoing dispersion and gravitational settlement is studied analytically. A general analytic expression for the dust concentration above the roughness height is derived. The steady state results obtained by previous authors are easily deduced from this expression. The profiles of the concentration are evaluated numerically and are found to be strongly dependent on time, height above the roughness surface, and falling speed. ¿1991 American Geophysical Union